US1951763A - Controls for boiler installations - Google Patents

Description

March 1934- H. c. MITTENDORF r AL 1,951,763

CONTROLS FOR BOILER INTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 1 a INVENTORS We? yam/u r masrfi March 1934- H. c. MITTENDORF El AL 1,951,763

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930- 4 Sheets-Sheet 2 INVEN OR a 4 5 Waxm er ATTORNEYJ March 20,1934. H. c. MITTENDORF ET-AL 1,951,763

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 3 INVENTOR MLLW ATTORNEYg March 20, 1934. H. c. MITTENDORF ET AL ,7

CONTROLS FOR BOILER INSTALLATIONS Filed June 27, 1930 4 Sheets-Sheet 4 mum,

I WMmjer/Y/ofor mmm' INVENTOR to the storage of coal in the mill occurring at Patented Mar. 20, 1934 UNITED STATES PATENT OFFICE CONTROLS FOR BOILER INSTALLATIONS Application June 27, 1930, Serial No. 464,132

11 Claims. (01. 110-400) such time, the mill becomes partlyplugged and although the coal feed rate may be quite low, even this amount of coal feed tends to aggravate the plugging condition. The plugging action within the mill isreflected at the exhauster by an increase suction due to the reduced air flow and the dropping of part of the coal which was in suspension back into the mill. At such times it is desirable to give the mill an opportunity to clear itself before any additional coal is added. From the above it will be seen that the suction at the exhauster is an indication of conditions occurring in the mill and we propose to utilize this suction to overcome difficulties herein fully pointed out.

For example, let us assume that the boiler is operating at a maximum rating and that the feeder, mill, and exhauster are running at their maximum speeds. Under this condition the feeder is supplying coal rapidly to the mill so that a considerable storage of coal takes place in the mill, which is desirable in order to prevent the mill from pounding at its high speed. Since the mill and exhauster, however, are being driven at their maximum speeds proper introduction of fuel into the furnace takes place. Let us assume that the boiler rating is now suddenly reduced to the point where the feeder, mill and exhauster are brought to their slowest speeds by the combustion control mechanism. The air flow which is governed by the exhauster speed then drops oil promptly and much of the fines which are in suspension drop back into the mill. Since the mill speed is also greatly reduced the plows of the mill will not throw up. the coal sufficiently to be picked up by the air for delivery to the exhauster and burner. It will be seen, therefore, that under the above conditions coal stores up in the mill, and as the feeder is still delivering coal to the mill, although at reduced speed, still more coal enters the mill to bring about an overload condition.

One of the primary objects of our invention is to overcome the above difliculties.

Under other conditions of operation there is a period in which insufficient coal is in the mill so that rumbling of the mill takes place. For example, with a sudden increase in the load taken from the boiler, the mill, exhauster, and feeder willbe speeded up by the combustion control 195 mechanism so that there would not be suflicient coal in the mill to prevent a sudden decrease in the suction drop through the mill, and while the control mechanism would eventually speed the feeder to the point where a proper level of coal This invention relates to controls for boiler installations and will be described in connection with installations in which the furnace is directly fired from a mill such as a roller mill, i. e., installations in which a feeder receives coal from a bin and delivers it to a pulverizer mill from which it is drawn by an exhauster and delivered into the furnace by means of suitable burners.

In such installations difliculties in firing are encountered because under certain conditions the mill becomes overloaded with coal. Under other conditions there is not suflicient coal in the mill to ensure proper operation. The conditions referred to will be understood from the following.

In steam generating installations of the character described it has been found that the mills tend to overload due either to a high percentage of fines being delivered into the mill at any one time, changes in the type of coal, or to a sudden reduction in boiler rating which finds the mill with more coal than is necessary for the reduced rating.

We are aware that automatic combustion controls have been employed in such installations in which the speeds of the various pieces of fuel handling equipment are changed to take care of the new load conditions, but we have found that there is a periodin which the mill is overloaded because of the storage of coal in it that took place when the boiler was operating at higher ratings and, therefore, even though the speeds of the devices be reduced at such a time, the overload condition will be further aggravated rather than helped.

Stated more fully the known automatic combustion controls do not use the positional idea. By this we mean that they do not take any deflnite position for any particular steaming rate of the boiler. They simply increase or decrease the speeds of the motors of the various pieces of fuel handling equipment whenever a change in pressure in the steam header line occurs. Consequently when the steaming rate changes the slight change in header pressure causes the controls to be moved to a new position which will again give the present headerpressure. While the motion of the control may be practically instantaneous it does not necessarily follow that the equipment can immediately adapt itself to the new conditions. Such a'case occurs when the steam demand is reduced over a wide range in a very short interval of time. The exhauster, pulverizer and feeder motors will be brought down to their slower speed positions, but, due

would exist in the mill, there nevertheless is a period in which the supply would be inadequate to prevent rumbling.

We propose, therefore, to provide a system of control whereby such rumbling is prevented.

We also contemplate the provision of a control system adapted to improve the control of boiler installations under conditions when there is a substantial drop of boiler rating from say 300% to 200% of rating.

Generally stated, therefore, it is an object of our invention to provide simple and effective control means for installations of the character described whereby proper coal conditions are ensured in the mill when changes in boiler rating take place.

A more specific object resides in the provision of eifective mill feeder control in boiler installations.

A further object resides in the provision of simple and effective control means in apparatus of the class described whereby approximately constant or predetermined condition of suction may be maintained at the exhauster under different operating conditions.

How the foregoing, together with such other objects and advantages as may hereinafter appear, or are incidentto our invention, are realized, is illustrated in preferred form in the accompanying drawings, wherein- Fig. 1 is an elevational view of a boiler installation embodying our invention; Fig. 2 is an enlarged elevational view of rheostat operating mechanism which we employ; Fig. 3 is 'an end view of Fig. 2, and Fig. 4 illustrates a simple form of wiring diagram.

The installation'illustrated comprises in gen eral a boiler A of any well known type, a pulverized coal burning furnace B having suitable burner means 0 for the introduction of the fuel, a pulverlzer mill D, to which coal is delivered by a feeder device E, a bin F for supplying coal to the feeder device, and an exhaust fan or exhauster G for drawing pulverized coal from the mill D for delivery into the furnace through the medium of the burner means C. A conduit 5 connects the mill to the suction side of the exhauster and a conduit 6 connects the discharge side of the exhauster to the burner C. Electric motors 7, 8 and 9 are employed for driving the feeder E, the mill D and the exhauster G respectively.

Control mechanism indicated as a whole by the reference letter H is provided for automatically controlling the feeder, mill, and exhauster motors. In general the control mechanism comprises three automatically operated rheostats 10, 11 and 12, one for each motor, the rheostat 10 being for the feeder motor 7, rheostat -11 for the mill motor 8 and rheostat 12 for the exhauster motor 9, and steam pressure actuated mechanism J for causing the automatic operation of the rheostats.

A cam shaft 13 .(see Figs. 2 and 3) carrying three cams 14, 15 and 16 is provided and the cams operate levers 17, 18 and 19 pivoted on a support 20. The lever 17 is actuated by the cam 14 and its movement is transmitted to the feeder rheostat 10 by means of a flexible chain or cable 21 which is secured at one end to the free end 22 of the lever 17 and carries a counterweight 23 at its other end. The flexible chain 21 passes around a sprocket wheel 24 secured on-the rheostat shaft 25 and then over an idler sprocket 26 from which the counterweight depends. Thus it will be seen that when the lever moves up and down the sprocket wheel 24 and rheostat 10 are correspondingly turned.

Inorder to initially set the rheostat or to correct its setting to a preselected position, we have provided a hand operated setting or correcting device 27 adapted to operate on the chain 21, which device comprises a member 28 secured on a rotatable shaft 29 having an operating handle 30, and a pair of sprockets 31, 31, rotatably carried by the member 28, between which the chain passes. When the handle 30 is rotated the run of chain between the lever 17 and sprocket wheel 24 is lengthened or shortened according to the direction in which the member 28 is rotated and this, in turn, rotates the sprocket 24 and rheostat 10 either in a clockwise or counter-clockwise direction to afford setting of the rheostat as desired. The device 27 may be locked in its adjusted position in any suitable manner.

Similar mechanisms indicated at 32 and 33 are employed for actuating the mill and exhauster rheostats l1 and 12 respectively, through operation of the levers 18 and 19 associated with the cams 15 and 16. Hand operated setting or correcting devices 34 and 35 are also provided in the mechanisms 32 and 33.

The cam shaft 13 is actuated by means of a Well known combustion control device indicated at J which is of the type receiving steam for its operation from the boiler. In general the device includes a pressure actuated piston 37 which is subjected to pressure under control of a valve device 38 operated by boiler steam pressure supplied thereto by a line 39 leading from a steam header 40 of the boiler. When the steam pressure in the line 39 increases, the piston 37 moves upwardly and this movement is transmitted to the cam shaft 13 by means of a link 41 connected at one end to the piston rod 42 and at its other end to a lever 43 secured to the cam shaft. The cam shaft is thus rotated in a direction to cause the cams 14, 15, and 16 to rock the levers 17, 18 and 19 upwardly and this in turn operates the rheostats in a manner to slow down the motors of the feeder, the mill, and the exhauster. When the steam pressure drops the reverse operation takes place and the motors are speeded up.

From the foregoing it will be seen that automatic control is provided whereby the motors of the various pieces of equipment are slowed down when there is an increase in steam pressure and speeded up when there is a steam pressure drop.

Under conditions of gradual decreases and increases in steam pressure the above system of control is quite satisfactory for controlling the firing of the furnace to suit changes in load and boiler rating. However, there are conditions in many installations where the changes are sudden and rapid which are not effectively taken care of.

This is particularly true in a steel plant where changes in rating are very rapid so that difficulties are encountered in firing due to conditions in the mill, as has been pointed out above.

In order to overcome such difficulties we supplement the above control device by suction operated devices adapted to additionally control the mill feeder as will now appear.

A pipe connection 44 is taken off the exhauster inlet elbow 45 or at some other convenient point in the mill where there is a maximum amount of suction, and is led to a well known type of suction relay device 46 having a pivoted lever 47 adapted to be rocked through the action of the suction on the relay. The pivoted lever 47 is connected, by means of a link 48, to any well known form of mercoid switch 49 or other type of switch adapted to make or break the feeder mo tor circuit. As will be clear from the wiring diagram illustrated in Fig. 4, .the mercoid switch 49 is connected into the feeder motor circuit, and, therefore, serves as a controlling medium to start or stop the feeder motor. 7, the speed of which is automatically regulated under normal conditions as hereinbefore described.

Referring to the wiring .diagram the circuit for the exhauster motor .9. is shown with the connections such that the motor is operating. Any suitable form of starting switch 50 may be employed. The rheostat 12. is shown in a position in which the motor field is weakened approximately two-thirds.

The circuit for the pulverizer motor 8 is similar to that just described and includes a suitable form of starting switch 51, therheostat 11 being shown in position to weaken the motor field approximately two-thirds.

The circuit for the feeder motor 7 is shown with the connections suchthat the motor is operating with its field weakened approximately two-thirds by the rheostat; 10. The automatic operation of the rheostats 10, 11 and 12 has been described hereinbefore,

The feeder motor 'I is remotely controlled by a suitable start stop switch 52 when the stop suction relay 46 is in the position corresponding to suction insufficient to cause it to open the control circuit through the medium of the mercoid switch 49. However, when the suction in the relay 46 reaches a predetermined value, the suction relay operates to rock the mercoid switch in a direction to open the mercury contact at 53 so that the circuit to a magnetic switch 54 is interrupted, thus causing the switch to, open. This opens the feeder motor control circuit-and stops the feeder motor until the mill has cleared itself and the suction at the inlet side of the exhauster has been again restored to its normal value. It is to be noted that with the parts in the position shown in the wiring diagram, the operation of the feeder motor 'I is similar to that of the motors 8 and 9 above described.

Assuming now that the boiler is operating at high rating with the feeder,- mill and axhauster running at their maximumspeeds and that the boiler rating is suddenly: reduced to the point where the motor rheostats 10, 11 and i2 immediately slow the motors to their slowest speeds.

then the suction relay 46 is operated on by high suction due to causes fully pointed out above and the pivoted lever 47 is moved in a direction to open the feeder motoncircuit, with the result that the feed to the mill is interrupted for a time and for such a length of time as it takes for the mill to be cleared of sufllcient coal to cause a reduction of the suction to. normal value. when this occurs the suction relay automatically is reset and the feeder motor circuit is closed to again start the feeder in operation which will then be under control of its rheostat 10.

when there is a sudden increase in the load taken from the boiler, the mill is speeded up suddenly under control of its-automatic rheostat 10, but we have found that there is not sufiicient coal in the mill at this instant to prevent a sudden decrease in the suction drop thru the mill, and, therefore, a period exists -when the mill would rumble extensively unless provision is made to get additional coal into the mill. That is to say, the

, rapid increase in air flow due to speeding up the exhauster motor through means of the combustion control mechanism J would tend to clear the mill quickly, and although the feeder motor is also accelerated to its maximum speed position as represented by the position of the rheostat 10, the suction will still be too low for a period of time so that it is desirable to accelerate the feed beyond that which would ordinarily be used for regulation. This situation is reflected at the exhauster inlet and, therefore, we propose to connect to the suction pipe 44, a second suction relay device 55, similar to the suction relay device 46. The suction relay device 55 is adapted to operate a mercoid switch 56 which, in turn, controls a fast magnetic relay 57. The fast relay S'lzcuts in or cuts out an additional field resistance 58 associated with the rheostat 10 as indicated in the wiring diagram. The contacts 59 of the magnetic relay 57 are normally closed and when the suction to the suction relay 55 drops to a predetermined value, it is operated to move the mercoid switch 56 to a position in which a mercury contact is made at 60 and this, in turn, causes the magnetic relay 57 to be energized, thus opening the contacts 59. When the contacts 59 are opened the extra resistance 58 is included in the field circuit and the feeder motor speeds up to its maximum speed. As soon as the suction builds up to the proper value, the fast suction relay 55 resets and deenergizes the magnetic relay 57, permitting the combustion control mechanism H and J to again act as the controlling medium in its usual fashion.

We also propose to improve the coal feed in installations of the character described by taking the change of feed in two steps under certain conditions, for example, when there is a drop in boiler rating from, say 300% to 200% of rating, i. e., a change of rating which is not severe enough to warrant completely interrupting the feed but substantial enough to warrant bringing the feed down to the minimum value for a short time. In

44, adapted to operate a mercoid switch 62. The L suction relay device 61 is set at a value slightly below the stop relay device 46 and actuates the mercoid switch 62 which, in turn, actuates a 1 slow magnetic relay 63 operating to reduce the motor speed to its minimum value. but without interrupting the motor circuit as will be clear from the wiring diagram.

The contacts 64 of the magnetic relay 63 are normally open and when the suction to the suction relay 61 reaches the predetermined value at which the relay is set, it is operated to move the mercoid switch 62 to a position in which a mercury contact is made at 65 and this, in turn, causes the magnetic relay 63 to be energized, thus closing the contacts 64 and putting the feeder motor field directly across the lines, thereby causing the motor to run at its minimum speed, no matter what position the rheostat is at under control of the mechanism J and H. When conditions become normal the mechanism J and H i Similar conditions may occur when the coal feeds irregularly due to changing fineness of the raw coal or hang up of the coal on account of moisture or other conditions. Such conditions would be corrected by the operation of the suction relays in addition to the speed changes effected by the combustion control mechanism referred to.

The following is an example as to the approximate values at which the suction relays would be set if it is desired to hold a suction of 4.25" water gage at the exhaust inlet. The fast suction relay 55 would be set so that the contacts 59 would open at 4.15" suction and close at 4.00" suction. The slow suction relay 61 would be set so that the contacts 64 would open at 4.35 suction and close at 4.50" suction. The stop suction relay 46 would be 'set so that the contacts 54 would open at 4.75" suction and close at 4.60" suction. With such settings approximate constant suction would be maintained at the exhaust inlet.

While the diagram illustrates the use of a D. C. feeder motor with regulation of the feed by adjustment of the shunt field, it is to be understood that we contemplate connecting the suction relays in a manner to control two motors or a two speed motor as would be the case in A. C. circuits. Also in some instances we may utilize a jumper connection" in place of the extra resistance 58.

From the foregoing it will be seen that we have provided a very simple, and effective control system for boiler,installations which operates to automatically control the feeder, mill, and exhauster speeds to ensure proper firing under normal changes in load conditions and also operates to additionally automatically control the feeder speed to ensure proper coal conditions in the mill when there is a sudden change in load.

While we have illustrated our invention in connection with a directly fired boiler installation, it is to be understood that the advantages to be derived therefrom may also be obtained in indirectly fired installations. Also in some installations very satisfactory results may be obtained by using the stop suction relay device 46 without the suction relay'devices 55 and 61.

We claim:

1. In combination of a boiler and furnace and means for firing the furnace, said firing means including a pulverizer, a fuel bin, a feeder for feeding fuel from the bin to the pulverizer, an exhaust fan for feeding pulverized fuel from the pulverizer to the furnace, automatic means for simultaneously controlling the speed of the feeder, the pulverizer and the exhaust fan responsive to variations of the steam pressure in the boiler, and automatic means independent of said first mentioned means for further controlling the suction of the exhaust fan. 7

2. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster for drawing pulverized coal from the mill for delivery to the furnace, automatic means responsive to variations of the steam pressure of the boiler for simultaneously controlling the speed of the mill, the feeder and the exhauster and automatic means independent of said first mentioned means for further controlling the speed of the feeder responsive to variations of the suction of the exhauster.

3. In a pulverized coal burning installation, the combination of a boiler; a furnace; a coal pulverizer, a coal feeder, and an exhaust fan, each having an electric driving motor; a rheostat controlling the speed of each motor; automatic means responsive to variations of the steam pressure speed of the feeder responsive to variations of the r rheostats, manually operating means for adjust ing said rheostats, and automatic means responsive to variations of the suction of the exhaust fan for controlling the speed of the feeder motor independently of the setting of its rheostat.

4. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster fan for drawing pulverized coal from the mill and delivering it to the furnace, automatic means for simultaneously controlling the speed of the mill, feeder, and fan responsive to variations of the steam pressure of the boiler, and automatic means independent of said first mentioned means for further controlling the speed of the feeder responsive to variations of the suction at the suction side of the fan.

5. In a boiler installation, the furnace of which is fired by direct delivery of fuel thereinto from a mill, the combination of a motor driven mill, a motor driven feeder for delivering fuel to the mill, a motor driven exhauster for drawing fuel from the mill and delivering it into the furnace, automatic means for simultaneously controlling the speed of said motors including a speed con troller for each motor, steam pressure actuated mechanism for operating said controllers, means for delivering actuating steam to said mechanism from the boiler, and other automatic means inde pendent of the first means for further controlling the speed of the feeder motor including means responsive to variations of the suction of the exhauster for controlling the speed of the feeder motor beyond the control afforded by the first mentioned control means.

6. In a boiler installation, the furnace of which is fired by direct delivery of fuel thereinto from a mill, the combination of a motor driven mill, a motor driven feeder for delivering fuel to the mill, a motor driven exhauster for drawing fuel from the mill and delivering it into the furnace, automatic means for simultaneously controlling the speed of said motors including a rheostat controlling each motor, means for operating the rheostats, steam pressure. actuated mechanism for actuating said operating means, means for delivering actuating steam to said mechanism from the boiler, and'other automatic means independ ent of said first mentioned means for further,controlling the speed of the feeder motor including a switch for said feeder motor, a suction relay adapted to open and close said switch, and a suction line leading fromthe suction side of the exhauster to said suction relay.

'7. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhausterfor drawing pulverized coal from the mill for delivery to the furnace, an electric driving motor for the mill, anelectric driving motor for the feeder, an electric driving motor for the exhauster, a rheostat in the mill motor circuit for controlling the speed of the mill motor, a rheostat in the feeder motor circuit for controlling the speed of the feeder motor, a rheostat in the exhauster motor circuit for controlling the speed of the exhauster motor, automatic means responsive to variations of the steam pressure of the boiler for simultaneously controlling the speed of said mill, feeder, and exhauster motors through the medium of their rheostats, and automatic means independent of said first means responsive to variations of the suction of the exhauster "for further controlling the operation of the feeder I of the boiler for simultaneously operating saidmotor including a magnetic switch in the feeder ,150

mamas motor circuit adapted to stop the motor, and a. switch for controlling said magnetic switch to stop and start the feeder motor.

8. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster for drawing pulverized coal from the mill for delivery to the furnace, an electric driving motor for the mill, an electric driving motor for the feeder, an electric driving motor for the exhauster, a rheostat in the mill motor circuit for controlling the speed of the mill motor, a rheostat in the feeder motor circuit for controlling the speed of the feeder motor, a rheostat in the exhauster motor circuit for controlling the speed of the exhauster motor, automatic means responsive to variations of the steam pressure of the boiler for simultaneously controlling the speed of said mill, feeder, and exhauster motors through the medium of their rheostats, and automatic means independent of said first mentioned automatic means responsive to variations of the suction of the exhauster for further controlling the operation of the feeder motor including a magnetic switch in the feeder motor circuit adapted to stop the motor, a magnetic switch in the field circuit of the feeder motor adapted to control the speed of the feeder motor by control of the field, a switch for controlling the first mentioned magnetic switch .to stop and start the feeder motor, and a switch for controlling the second mentioned magnetic switch to vary the speed of the feeder motor.

9. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster for drawing pulverized coal from the mill for delivery to the furnace, an electric driving motor for the mill, an electric driving motor for the feeder, an electric driving motor for the exhauster, a rheostat in the mill motor circuit for controlling the speed of the mill motor, a rheostat in the feeder motor circuit for controlling the speed of the feeder motor, a rheostat in the exhauster motor circuit for controlling the speed of the exhauster motor, automatic means responsive to variations of the steam pressure of the boiler for simultaneously controlling the speed of said mill, feeder, and exhauster m0- tors through the medium of their rheostats, and automatic means independent of said first mentioned automatic means responsive to variations of the suction of the exhauster for further controlling the operation of the feeder motor including a magnetic switch in the field circuit of the feeder motor adapted to cause the feeder motor to speed up by control of the field thereof, and

a switch for controlling said magnetic switch to speed up the feeder motor.

10. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster for drawing pulverized coal from the mill for delivery to the furnace, an electric driving motor for the mill, an electric driving motor for the feeder, an electric driving motor for the exhauster, a rheostat in the mill motor circuit for controlling the speed of the mill motor, a rheostat in the feeder motor circuit for controlling the speed of the feeder motor, a rheostat in the exhauster motor circuit for controlling the speed of the exhauster motor, automatic means responsive to variations of the steam pressure of the boiler for simultaneously controlling the speed of said mill, feeder, and exhauster motors through the medium of their rheostats, and automatic means independent of said first mentioned automatic means responsive to variations 7 of the suction of the exhauster for further controlling the operation of the feeder motor including a magnetic switch in the field circuit of the feeder motor adapted to cause the feeder motor to slow down by control of the field thereof, and a switch for controlling said magnetic switch to slow down the feeder motor.

11. A pulverized coal burning installation including a boiler, a furnace, a mill for pulverizing the coal, a feeder for delivering coal to the mill, an exhauster for drawing pulverized coal from the mill for delivery to the furnace, driving means for the mill, driving means for the feeder, driving means for the exhauster, automatic means for simultaneously controlling the speeds of the said driving means responsive to variations of the steam pressure of the boiler, an automatic means independent of said first mentioned automatic means for further controlling the speed of the driving means for the feeder including a plurality of devices responsive to variations of the suction of the exhauster each adapted to be set to operate at predetermined values of suction, means associated with one of said devices adapted to stop the feeder driving means when the suction reaches a predetermined value, means associated with another of said devices adapted to reduce the speed of the feeder driving means when the suction reaches a predetermined value lower than that at which said first device operates and means associated with another of said devices adapted to increase the speed of the feeder driving means when the suction drops to a value below that at which the other devices operate.

HARVEY C. MI'I'I'ENDORF. ALBERT C. FOSTER.

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